DESCRIPTION: Orthodontic root resorption is a major clinical and medico-legal concern. Approximately half the US population is treated orthodontically, and according to some estimates, 90 percent of patients experience detectable root-loss. Five to 58 percent experience severe (>5mm) loss. The mechanisms remain obscure, but it is believed that osteoclasts are responsible for this genetics-modulated loss. Ultimately, resorption is a response to loads on the crowns of teeth. When orthodontic forces are superimposed on altered occlusion, the "natural" root deformations are changed in magnitude, direction, duration, and frequency. It is proposed that these deformations cause root damage in two ways. First, as the root flexes, the convex bending side is stretched, while the other (concave) side is compressed. This tension- /compression can damage the cementum and/or dentin. Second, root bending also produces shear stresses within the cementum-dentin interface, leading to possible delamination. It is further hypothesized that normal physiological resorption sites act as stress concentrators that intensify local root damage. In turn, the damaged root is more vulnerable to resorption. These phenomena will be examined using mechanical testing, histology, a novel application of microCT, and finite element (FE) computer models. [unreadable] [unreadable] The Specific Aims of this study are to (1) quantify root fatigue damage, (2) develop/validate microCT damage assessment, (3) correlate stress gradients with damage, (4) relate root-shape to stresses, and (5) assess damage in the adjacent contralateral root to determine its suitability for use as control. [unreadable] [unreadable] Cyclic loading will be applied to the crowns of fresh cadaveric canine (dog) mandibular central incisors. Blocks with a tooth and its surrounding alveolar bone will be microCT imaged and then examined for damage using the traditional basic fuchsin histological method. The two damage assessments will be compared and the locations of damage will be correlated with the results of FE calculated stress gradients. This multidisciplinary project will contribute in two disparate areas - understanding the mechanism of orthodontics-associated root resorption, and demonstrating the potential of 3D microCT imaging to become the new "gold standard" of calcified tissue damage assessment. [unreadable] [unreadable]